This proposal seeks to develop a novel method for controlling the activity of engineered proteins within cells. The use of chemical dimerizing agents such as rapalog have been very valuable as a means of activating cell surface receptors or disabling proteins in a cell by crosslinking them. We seek to create a Chemically-Induced Dimerization agent (CID) that is cleaved by light, thereby making the dimerization rapidly reversible. Such a molecule needs to lack endogenous binding partners in cells, be readily membrane permeant, have no toxic side effects, and be efficiently cleaved by light of a wavelength that will not cause cellular damage or interfere with standard imaging methods. We propose to characterize a candidate photocleavable CID to determine its kinetics and concentration dependence and thereby validate its suitability for work in cells in culture. We propose methods to optimize its affinity and efficacy, and we propose to test the reagent in a split kinesin assay of axonal transport to determine if it can be used to rapidly uncouple a cargo from its anterograde motor in a live neuron.